Heavy-load tire with wind bead structure

ABSTRACT

A heavy-load tire has a wind bead structure in which a turned-up portion of a carcass ply is wound around a bead core, in which a bead portion is equipped with a bead reinforcing layer having a u-shaped cross section and a bead apex rubber having a triangular-shaped cross section. The turned-up portion has an auxiliary turned-up portion passing through the vicinity of a radially outer side of the bead core. The bead apex rubber includes a high complex elasticity modulus inner apex portion disposed at a radially inner side and a low complex elasticity modulus outer apex portion disposed at a radially outer side.

This application is a Divisional of application Ser. No. 12/149,239,filed on Apr. 29, 2008 , now U.S. Pat. No. 7,997,318, issued Aug. 16,2011. Priority is also claimed to Japanese Application No. 2007-183442filed on Jul. 12, 2007 and Japanese Application No. 2007-128167 filed onMay 14, 2007. The entire contents of each of these applications ishereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a heavy-load tire with improveddurability of a bead portion.

BACKGROUND OF THE INVENTION

Conventionally, as shown in FIG. 12, various heavy-load tires having aso-called wind bead structure in which a turned-up portion b2 of acarcass ply b is wound around a bead core c have been proposed. Such aheavy-load tire can prevent damages arising from an end portion of theturned-up portion b2 because the end portion of the turned-up portion b2is disposed near the bead core c, which is less susceptible todistortion during driving. As a result, such a heavy-load tire has anoutstanding advantage in that the bead durability can be improved.

with the wind bead structure, the bead deformation at the time ofgrounding is relatively large as compared with a conventional non-windbead structure. Therefore, a bead reinforcing layer f becomes necessarywhich has a reinforcing cord such as a steel cord and which reinforces abead portion by being turned up around the bead portion into a U shape.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, when such a bead reinforcing layer f is provided, a largeshearing force acts on an outer end ft of an outer piece portion f1 ofthe bead reinforcing layer f, and thus, for example, rubber separation(cord loose) is likely to occur at the outer end ft. Thus, there is roomfor further improvement for the bead durability.

The present invention aims to provide a heavy-load tire having a windbead structure which can improve the bead durability.

Means for Solving the Problem

According to a first aspect of the invention, a heavy-load tireincludes: a carcass having one carcass ply having a body portionextensive from a tread portion through a side wall portion to a beadcore in a bead portion and a turned-up portion which extends from thebody portion, which is turned up from an axially inner side to anaxially outer side over the bead core, and which has an auxiliaryturned-up portion; a bead reinforcing layer which is disposed at thebead portion and has a reinforcing cord; and a bead apex rubber which isdisposed farther radially outwardly than the auxiliary turned-up portionof the carcass ply and which extends radially outwardly in a taperingmanner.

The turned-up portion has: a main turned-up portion which curves alongan axially inner side, a radially inner side, and an axially outer sideof the bead core; and the auxiliary turned-up portion which extends fromthe main turned-up portion and extends toward the body portion through avicinity of a radially outer side of the bead core.

The bead reinforcing layer has a u-shaped cross section including: anintermediate portion which extends farther radially inwardly than themain turned-up portion and along the main turned-up portion; an outerpiece portion which extends from an axially outer side of theintermediate portion and which extends radially outwardly away from theturned-up portion; and an inner piece portion which extends from anaxially inner side of the intermediate portion and which extendsradially outwardly along the axially inner side of the body portion.

The reinforcing cord of the bead reinforcing layer has a cord strengthof 700 to 1200 N and the reinforcing cord inclines at an angle of 40 to70° relative to a circumferential direction of the tire at the outerpiece portion.

The bead apex rubber has: an inner apex portion which is formed of ahigh complex elasticity modulus rubber and is disposed at a radiallyinner side; and an outer apex portion which is formed of a rubber whosecomplex elasticity modulus is lower than that of the rubber of the innerapex portion and is disposed at a radially outer side.

The inner apex portion has an L-shaped cross section including: a bottompiece portion along a radially outer side of the auxiliary turned-upportion; and a raised piece portion which rises at an axially inner endside of the bottom piece portion and extends radially outwardly in atapering manner along the body portion of the carcass ply.

According to a second aspect of the invention, a heavy-load tireincludes: a carcass having one carcass ply having a body portionextensive from a tread portion through a side wall portion to a beadcore in a bead portion and a turned-up portion which extends from thebody portion, which is turned up from an axially inner side to anaxially outer side over the bead core, and which has an auxiliaryturned-up portion; and a bead reinforcing layer which is disposed at thebead portion and has a reinforcing cord.

The turned-up portion has: a main turned-up portion which curves alongan axially inner side, a radially inner side, and an axially outer sideof the bead core; and the auxiliary turned-up portion which extends fromthe main turned-up portion and extends toward the body portion through avicinity of a radially outer side of the bead core.

The bead reinforcing layer has a U-shaped cross section including: anintermediate portion which extends farther radially inwardly than themain turned-up portion and along the main turned-up portion; an outerpiece portion which extends from an axially outer side of theintermediate portion and which extends radially outwardly away from theturned-up portion; and an inner piece portion which extends from anaxially inner side of the intermediate portion and which extendsradially outwardly along the axially inner side of the body portion.

The bead portion is provided with a heel cover layer formed of a cordply having an organic fiber cord at least one portion of an outer sideof a heel area defined below, and the organic fiber cord of the heelcover layer is arranged at an angle of 30 to 90° relative to acircumferential direction of a tire.

The heel area is defined as an area sandwiched between a first straightline extending radially inwardly in a perpendicular manner to a bottomsurface of the bead portion from a cross section center of the bead coreand a second straight line extending axially outwardly in aperpendicular manner to the first straight line from the cross sectioncenter of the bead core.

Effects of the Invention

In the heavy-load tire according to a first aspect of the invention, thereinforcing cord having a cord strength as high as 700 to 1,200 N isused for the bead reinforcing layer, and the reinforcing cord in theouter piece portion inclines at an angle of 40 to 70° relative to thecircumferential direction of the tire, thereby increasing the effect ofreinforcing. Such a structure increases flexural rigidity of the beadportion to thereby improve driving stability. However, the stressconcentrated on the outer end of the outer piece portion increases,which causes a tendency that rubber separation at the outer end ispromoted.

The stress concentration at the outer end can be reduced by the use ofthe bead apex rubber which has the inner apex portion having an L-shapedcross section. Specifically, in the bead apex rubber, the inner apexportion formed of a high complex elasticity modulus rubber has anL-shaped cross section including the bottom piece portion along theauxiliary turned-up portion and the raised piece portion along the bodyportion of the carcass ply. This secures the flexural rigidity of thebead portion and maintains the driving stability while making small thevolume of the high complex elasticity modulus rubber. Furthermore, therubber volume of the outer apex portion formed of a low complexelasticity modulus rubber can be increased in exchange for the reductionin the volume of the high complex elasticity modulus rubber. As aresult, the shear distortion which acts on the outer piece portion ofthe bead reinforcing layer adjacent to the outer apex portion can besufficiently eased. Thus, the rubber separation at the outer end of theouter piece portion can be effectively suppressed to thereby increasethe bead durability.

According to the heavy-load tire of the second aspect of the invention,the heel cover layer which is composed of a cord ply in which an organicfiber cord is arranged at a predetermined angle is provided at the outerside of the heel area of the bead portion.

Here, in the wind bead structure, since the carcass ply is wound aroundthe bead core, the cord tension of the carcass ply during driving iseasily transmitted to the bead core as compared with a conventional tirehaving a non-wind bead structure. As a result, there is a tendency thatthe bead core is likely to move to the side of a rim flange under theeffect of the cord tension and a high charging internal pressure. Withthe movement of the bead core, the movement of the body portion of thecarcass ply also becomes large. As a result, the stress concentrating atthe outer end of the outer piece portion of the bead reinforcing layerincreases, and thus the rubber separation is promoted. Moreover, withthe movement of the bead core, a chafer rubber (referred to as a clinchrubber) is sandwiched between the bead core and a rim to be compressedin the thickness direction. However, the chafer rubber extends in adirection perpendicular to the thickness direction. The extension causesdistortion at the interface between the bead reinforcing layer and thechafer rubber, whereby rubber separation is induced.

However, the heel cover layer according to a second aspect of theinvention is unified with the chafer rubber, thereby suppressing theextension of the chafer rubber in the direction perpendicular to thethickness direction. Therefore, the separation between the beadreinforcing layer and the chafer rubber can be suppressed. Moreover, asa result that the compression deformation of the chafer rubber in thethickness direction is suppressed by the suppression of the extension,the movement of the bead core toward the rim flange side is suppressed,whereby the movement of the body portion of the carcass ply is reduced.Furthermore, since the heel cover layer is unified with the chaferrubber to thereby improve the flexural rigidity, bead deformation can besuppressed, which is combined with the reduction in the movement of thebody portion to suppress the rubber separation at the outer end of theouter piece portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, an embodiment of the present invention will be described inconjunction with the drawings in which:

FIG. 1 is a cross sectional view illustrating an embodiment of aheavy-load tire according to a first aspect of the present invention.

FIG. 2 is an enlarged cross sectional view illustrating a bead portionof the heavy-load tire according to the first aspect of the presentinvention.

FIG. 3 is a further enlarged cross sectional view illustrating anessential portion of the bead portion.

FIG. 4 is a side cross sectional view of the bead portion illustratingthe arrangement of reinforcing cords in an outer piece portion of a beadreinforcing layer.

FIG. 5 is a cross sectional view illustrating an inner apex portion.

FIG. 6 is a cross sectional view illustrating a heel cover layer.

FIG. 7 is a partial side view of the bead portion illustrating thearrangement of organic fiber cords of the heel cover layer.

FIG. 8 is a partial side view of the bead portion illustrating anotherarrangement of the organic fiber cords of the heel cover layer.

FIG. 9 is a cross sectional view illustrating another example of thebead portion of the heavy-load tire according to the first aspect of theinvention.

FIG. 10 is a cross sectional view illustrating an embodiment of aheavy-load tire according to a second aspect of the invention.

FIG. 11 is an enlarged cross sectional view illustrating a bead portionof the heavy-load tire according to the second aspect of the invention.

FIG. 12 is a partial cross sectional view of a conventional beadportion.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 to 9 illustrate one embodiment of a heavy-load tire 1A accordingto a first aspect of the invention. FIGS. 10 and 11 illustrate oneembodiment of a heavy-load tire 1B according to a second aspect of theinvention.

Unless otherwise specified, the dimensions and the like of the parts ofthe tire are determined as those measured when the tire is mounted on anormal rim J and a normal pressure is charged, which is a no-load normalinternal pressure condition. The term “standard rim”, as used herein,refers to a rim specified as corresponding to the tire in a standardsystem encompassing the standard upon which the tire is based. Forexample, the standard rim is the “standard rim” specified in JATMA,“Design Rim” specified in TRA, or “Measuring Rim” specified in ETRTO.The term “standard pressure”, as used herein, refers to an air pressurespecified as corresponding to the tire in a standard system encompassingthe standard upon which the tire is based. For example, the standardpressure is the maximum air pressure in JATMA, the maximum pressuregiven in the “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES”table in TRA, or the “INFLATION PRESSURE” in ETRTO.

The heavy-load tire 1A is equipped with a carcass 6 in the shape of atoroid which extends from a tread portion 2 to a bead core 5 in a beadportion 4 through a sidewall portion 3 and a belt layer 7 which isdisclosed at a radially outer side of the carcass 6 and the inner sideof the tread portion 2.

The belt layer 7 is composed of at least two belt plies using belt cordseach composed of steel cords. In this example, three belt plies 7A to 7Care provided. The belt cords of at least two belt plies among the beltplies 7A to 7C are disposed at a small angle (e.g., 10 to 35° relativeto the circumferential direction of the tire and in such a manner thatthe plies cross each other. It should be noted that the belt layers 7may be composed of four or more belt plies.

The carcass 6 is composed of a carcass ply 6A having a body portion 6 ain the shape of a toroid which extends from the tread portion 2 to thebead core 5 in the bead portion 4 through the sidewall portion 3 and aturned-up portion 6 b which extends from the body portion 6 a and isturned up from an axially inner side to an axially outer side over thebead core 5. The carcass ply 6A has a carcass cord formed of a steelcord disposed at an angle of 80 to 90° relative to a tire equator C.

The bead core 5 is a ring material which has an oblong flat hexagon-likecross section formed by winding a bead wire 5 w formed of steel in sucha manner as to form multiple layers and multiple rows as enlargedlyillustrated in FIG. 3. Specifically, the bead core 5 has an inner sideSL serving as a long side of the radially inner side in the hexagon-likecross section, an outer side SU serving as a long side of the radiallyouter side, an inner side Si having a chevron-shaped curved side whichconnects the inner side SL to the outer side SU at the axially innerside, and an outer side so serving as a curved side of the oppositeside. The bead core 5 is surrounded by a so-called lapping rubber 16,which prevents direct contact with the carcass cord.

In this example, the radially inner side SL of the bead core 5 extendsalmost in parallel with a sheet surface J1 (illustrated in FIG. 1) ofthe rim J. Thereby, high fitting force between the bead portion 4 andthe rim is obtained over a wide range. In this example, since the rim Jis a 15° tapered rim, the radially inner side SL and the radially outerside SU of the bead core 5 are inclined at about 15° relative to animaginary axial line.

Moreover, as illustrated in FIG. 3, the turned-up portion 6 b of thecarcass ply 6A has a so-called a wind bead structure composed of a mainturned-up portion 6 b 1 which smoothly curves along an axially innerside Si, a radially inner side SL, and an axially outer side so of thebead core 5, and an auxiliary turned-up portion 6 b 2 which extends fromthe main turned-up portion 6 b 1 and extends toward the body portion 6 athrough the vicinity of the radially outer side SU of the bead core 5.As a preferable aspect, it is preferable that the turned-up portion 6 bsmoothly curve circularly without having an angled corner that islocally bent. It should be noted that when the turned-up portion 6 b hasan angled corner, there is a tendency that the strength of the carcasscord is reduced.

The auxiliary turned-up portion 6 b 2 is defined as a portion at aradially outer side relative to an extension K which is obtained byextending the radially outer side SU of the bead core 5. The auxiliaryturned-up portion 6 b 2 extends and is inclined in a direction in whichthe distance from the extension K increases toward an outer end 6 bt.The angle θ to the extension K of the auxiliary turned-up portion 6 b 2is preferably 10° or more, and more preferably 15° or more. Thissuppresses the local bending of the carcass cord 6C. If the angle θ istoo large, the suspension ability to the bead core 5 of the turned-upportion 6 b becomes weak. This is likely to produce a phenomenon thatthe turned-up portion 6 b is drawn to the body portion 6 a side (aso-called a blow-by phenomenon). In view of the above respects, theangle θ is preferably 60° or less, more preferably 45° or less, andstill more preferably 40° or less.

It should be noted that the angle θ is defined as an angle formed by theextension K and a straight line N, which connects a point F where thecarcass cord 6 c of the turned-up portion 6 b crosses the extension K ofthe bead core 5 and the outer end 6 bt of the turned-up portion 6 b.Here, in some cases, the radially outer side SU of the bead core 5becomes a non-flat surface due to, for example, variation in thepositions of the bead wires 5 w. In such a case, the extension K isapproximated by a tangent which touches a bead wire 5 wo disposed at anaxially outermost side and a bead wire 5 wi disposed at an axiallyinnermost side among bead wire rows appearing at the radially outer sideSU of the bead core 5.

Moreover, the shortest distance U1 between the outer end 6 bt of theturned-up portion 6 b and the extension K is preferably 2.0 mm or more,and more preferably 3.0 mm or more. If the distance U1 is less than 2.0mm, the carcass cord needs to be sharply bent, which easily causes areduction in the cord strength. If the distance U1 is excessively large,there is a tendency that the stress at the time of bending deformationof the bead portion 4 concentrates at the outer end 6 bt, which is notpreferable. In view of the respects, the distance U1 is preferably 8.0mm or less, and more preferably 6.0 mm or less.

Moreover, in the carcass cord 6C, the outer end 6 bt of the turned-upportion 6 b terminates short of the body portion 6 a instead of reachingthe body portion 6 a. The shortest distance U2 between the outer end 6bt and the body portion 6 a is preferably 0.5 mm or more, and morepreferably 1.0 mm or more. If the distance U2 is less than 0.5 mm,friction may occur between the outer end 6 bt and the body portion 6 a,which may cause fretting breakage depending on how the bead portion 4 fis deformed. In contrast, if the distance U2 is excessively large, thesuspension ability of the turned-up portion 6 b to the bead core 5 islikely to become insufficient, and thus the above-mentioned blow-byphenomenon easily arises. In view of the respects, the distance U2 ispreferably 5.0 mm or less, and more preferably 4.0 mm or less.

Furthermore, in the heavy-load tire 1A, in order to prevent spring backof the auxiliary turned-up portion 6 b 2, an auxiliary cord layer 8 isprovided radially outwardly relative to the auxiliary turned-up portionb2. This auxiliary cord layer 8 is formed of a ring-shaped material inwhich an auxiliary cord 8 w formed of, for example, steel cord, isspirally wound in the circumferential direction of the tire at leastonce, preferably two or more times. Thus, the spring back can bereliably suppressed without excessively molding the carcass load 6 c.Therefore, the auxiliary turned-up portion 6 b 2 can be stablymaintained in an intended form while preventing a sharp reduction in thestrength of the carcass cord resulting from molding.

As the auxiliary cord 8 w, a steel cord whose cord strength is 2000 to4000 N is preferable. If the cord strength is less than 2000 N, there isa tendency that the holding effect for the auxiliary turned up portionb2 becomes insufficient. If the cord strength exceeds 4000 N, there is atendency that the auxiliary cord 8 w is stiffened, which makes itdifficult to wind the cord.

Moreover, as illustrated in FIGS. 1 and 2, the bead portion 4 isprovided with a bead reinforcing layer 9 having a u-shaped cross-sectionand a bead apex rubber 10 having a substantially triangular-shapedcross-section disposed radially outwardly relative to the auxiliaryturned-up portion b2.

As shown in FIG. 2, the bead reinforcing layer 9 is composed of anintermediate portion 9 a circularly extending farther radially inwardlythan the main turned-up portion 6 b 1 and along the main turned-upportion 6 b 1, an outer piece portion 9 o which extends from the axiallyouter side of the intermediate portion 9 a and radially outwardlyextends away from the main turned-up portion 6 b 1 , along an axiallyouter side of the bead apex rubber 10 and an inner piece portion 9 iwhich extends from the axially inner side of the intermediate portion 9a and extends radially outwardly along the axially inner side of thebody portion 6 a of the carcass ply 6A. The bead reinforcing layer 9 isformed of one ply in which the reinforcing cords 9C each formed of steelcord are arranged.

With the wind bead structure, since the turned-up portion 6 b of thecarcass ply 6A is wound around the bead core 5, the flexural rigidity ofthe bead portion 4 becomes insufficient as compared with a conventionaltire of a non-wind bead structure, in which the turned-up portionextends radially outwardly along the outer side of the bead apex rubber.Moreover, during vulcanization, rubber including a bead apex rubber andthe like is likely to axially outwardly flow non-uniformly and broadly.As a result, there is a tendency that variation in a rubber gage arisessuch as reduction in thickness of the rubber gage of the bead portion.

Therefore, the bead portion 4 is provided with the bead reinforcinglayer 9 having a u-shaped cross section including the outer pieceportion 9 o and the inner piece portion 9 i. This improves the flexuralrigidity of the bead portion 4, thereby securing driving stabilitycomparable to that of a conventional tire having a non-wind beadstructure. Further, the outer piece portion 9 o suppresses a rubber flowat the bead portion during vulcanization and thus stabilizes the rubbergage of the bead portion, thereby preventing the variation in the rubbergage.

Here, the cord strength of the reinforcing cord 9C of the beadreinforcing layer 9 is at least 700 N or higher, preferably 800 N orhigher, and more preferably 850 N or higher. If the cord strength isless than 700 N, there is a possibility that the reinforcing cord 9C maybe susceptible to fracture and plastic deformation due to deformation ofthe bead portion 4 during driving and also there is a possibility thatthe cord is likely to deform by a pressure applied from a bladder duringvulcanization. In contrast, if the cord strength becomes excessivelylarge, there is a possibility that molding to obtain a u shape crosssection becomes difficult to carry out, which incurs a sharp increase inmanufacturing cost. In view of the respects, the cord strength of thereinforcing cord is at least 1200 N or less, preferably 1100 N or lower,and more preferably 1000 N or lower. It should be noted that the cordstrength of the auxiliary cord 8 w is higher than the cord strength ofthe reinforcing cord 9C.

FIG. 4 is a side cross sectional view of the bead surface in which therubber at the outer side of the bead portion 4 is removed so that theouter piece portion 9 o of the bead reinforcing layer 9 can be seen fromthe axially outer side. In the outer piece portion 9 o, the reinforcingcord 9C of the bead reinforcing layer 9 is disposed inclining at anangle α of 40 to 70° relative to the circumferential direction of thetire. If the angle α is less than 40°, the reinforcing cord 9C of thebead reinforcing layer 9 does not exhibit sufficient resistance againsttilt of the bead portion 4 toward the axially outer side or the pressureduring vulcanization. In contrast, if the angle α exceeds 70°, theangular difference β between the reinforcing cord 9C of the beadreinforcing layer 9 and the carcass cord 6C of the body portion 6 a ofthe carcass ply 6A becomes small, which causes a tendency that asufficient effect of reinforcing cannot be achieved. In view of therespects, the lower limit of the angle α is preferably 45° or morerelative to the circumferential direction of the tire, and the upperlimit thereof is preferably 65° or lower, and more preferably 60° ormore.

Moreover, in order to improve the flexural rigidity of the bead portion4 more effectively with the bead reinforcing layer 9, it is preferablethat each of the height hi and ho from a bead base line BL of the outerpiece portion 9 o and the inner piece portion 9 i is 15 mm or more, andpreferably 20 or more, as illustrated in FIG. 2. In contrast, if each ofthe height ho and the height hi is excessively large, the tire weight isincreased and the durability is degraded due to that the outer end ft ofthe outer piece portion 9 o comes closer to the sidewall portion 3 whichsharply bends during driving. In view of the respects, each of theheight ho and the height hi is preferably 40 mm or lower, and morepreferably 35 mm or lower.

In particular, hi>ho is preferable. Thereby, the stress concentration atthe outer end ft of the outer piece portion 9 o during driving isreduced. In view of the respects, the difference between the heights(hi−ho) is preferably 2 mm or more. Since the inner piece portion 9 i isdisposed adjacent to the body portion 6 a, the stress which acts on theouter end is small as compared with the outer piece portion 9 o. Thus,breakage at the outer end of the inner piece portion 9 i is less likelyto occur.

Next, the bead apex rubber 10 is composed of an inner apex portion 10Awhich is formed of a high complex elasticity modulus rubber and disposedradially inwardly and an outer apex portion 10B which is formed of arubber whose complex elasticity modulus is lower than that of the rubberof the inner apex portion 10A and disposed radially outwardly. Moreover,the inner apex portion 10A has an L-shaped cross section including abottom piece portion 10A1 along the radially outer side of the auxiliaryturned-up portion 6 b 2 and a raised piece portion 10A2 which rises atthe end of the radially inner side of the bottom piece portion 10A1 andextends radially outwardly in a tapering manner along the body portionof the carcass ply 6A.

Thus, the inner apex portion 10A formed of a high complex elasticitymodulus rubber has an L-shaped cross section including the raised pieceportion 10A2 and the bottom piece portion 10A1 and the raised pieceportion 10A2 radially extends along the outer side of the body portion 6a of the carcass ply. As a result, high flexural rigidity isdemonstrated against the tilt of the body portion 6 a of the carcass ply6A toward the rim flange side, which cooperates with the beadreinforcing layer 9 to maintain the driving stability, and moreover,improve the driving stability. Moreover, since the inner apex portion10A has an L-shaped cross section, the rubber volume of the outer apexportion 10B formed of a low complex elasticity modulus rubber can beincreased in exchange for a reduction in the rubber volume of the highcomplex elasticity modulus rubber. This s sufficiently alleviates theshear distortion which acts on the outer end portion of the outer pieceportion 9 o of the bead reinforcing layer 9, which prevents the rubberseparation at the outer end ft of the outer piece portion 9 o to therebyimprove the bead durability. It should be noted that since the highcomplex elasticity modulus rubber has relatively high energy loss, theheavy-load tire 1A can reduce rolling resistance. Furthermore, since thethickness of the raised piece portion 10A2 gradually decreases radiallyoutwardly, the difference in rigidity between the inner apex portion 10Aand the outer apex portion 10B is reduced. This leads to prevention ofbreakage arising at the radially outer end of the raised piece portion10A2 and the like.

A high complex elasticity modulus rubber having a complex modulus ofelasticity E*1 of 20 to 70 MPa is preferably used for the inner apexportion 10A. If the complex modulus of elasticity E*1 is less than 20MPa, the effect of reinforcing the flexural rigidity of the bead portion4 relatively decreases. In contrast, if the complex modulus ofelasticity E*1 exceeds 70 MPa, the ability to alleviate distortion islowered, and the flexural rigidity of the bead portion 4 excessivelyincreases, which causes a possibility of considerably degradingcomfortable ride. In particular, the lower limit of the complex modulusof elasticity E*1 of the inner apex portion 10A is preferably 35 MPa ormore and the upper limit thereof is preferably 60 MPa or less. For theouter apex portion 10B, a low complex elasticity modulus rubber whosecomplex modulus of elasticity E*2 is 2.0 to 6.0 MPa is preferably used.If the complex modulus of elasticity E*2 is less than 2.0 MPa, theeffect of reinforcing the bead portion 4 becomes insufficient. Incontrast, when the complex modulus of elasticity E*2 exceeds 6.0 MPa,the ability to alleviate distortion becomes insufficient.

The complex moduli of elasticity E*1 and the complex modulus ofelasticity E*2 refer to values measured at a temperature of 70° C., at afrequency of 10 Hz, at an initial distortion of 10%, and at an amplitudeof 2% using a viscoelasticity spectrometer manufactured by IWAMOTOSEISAKUCHO CO., LTD.

Moreover, the height Hb of the outer apex portion 10B (i.e., radialheight from the bead base line BL to the radial outer end of the outerapex portion 10B) is preferably 40 to 100 mm. The height Ha of theraised piece portion 10A2 (i.e., radial height from the bead base lineBL to the radially outer end of the raised piece portion 10A2) is notless than 35 mm and less than the height Hb.

If the height Hb is less than 40 mm or the height Ha is less than 35 mm,the flexural rigidity of the bead portion 4 cannot be sufficientlyimproved, and thus there is a possibility that the driving stability maybe lowered. In contrast, if the height Hb exceeds 100 mm or f the heightHa is equal to or more than the height Hb, the bead apex rubber 10 isunnecessarily enlarged, resulting in an increase in tire weight.Moreover, the outer end of the inner apex portion 10A and the outer apexportion 10B are aligned to each other, whereby distortion concentratesthere, resulting in that breakage is likely to occur. In order toimprove the flexural rigidity of the bead portion 4 more effectivelywithout degrading the durability, the height Ha of the outer end of theinner apex portion 10A is preferably larger than the height hi of theinner piece portion 9 i of the bead reinforcing layer 9.

Furthermore, the thickness of the raised piece portion 10A2 graduallydecreases radially outwardly. It is preferable that the thickness Ta ofthe raised piece portion 10A2 is adjusted to 1.0 to 4.0 mm and thethickness Tb of the outer apex portion 10B is adjusted to 7.0 to 13.0 mmon the base line X1 defined below. The base line X1 is defined as a lineorthogonal to the outer side of the tire through the point P which isdisposed at the axially outer side of the bead apex rubber 10 and whichhas a distance of 25 mm from the bead base line BL toward the radiallyouter side. As shown in FIG. 2, the outer piece portion 9 o of the beadreinforcing layer 9 contacts the point P, and both the outer pieceportion 9 o and the inner piece portion 9 i of the bead reinforcinglayer 9 extend radially outward from the base line X1.

The height position having a distance of 25 mm from the bead base lineBL is near the radially outermost position in a region in which the tireouter side is in contact with the rim fringe when a normal (standard)load is applied to the tire in a condition of a normal inner pressure,and, at the position, a large distortion is likely to occur. Therefore,the durability of the bead portion 4 can be more effectively increasedby specifying the thicknesses Ta and Tb of such a height position.

If the thickness Ta of the raised piece portion 10A2 is less than 1.0mm, the flexural rigidity of the bead portion 4 is lowered and thedriving stability is lowered. In contrast, if the thickness Ta exceeds4.0 mm, the rubber volume of the outer apex portion 10B decreases, andthus the ability to alleviate distortion becomes insufficient. In viewof the respects, the lower limit of the thickness Ta of the raised pieceportion 10A2 is preferably 1.5 mm or more and the upper limit thereof ispreferably 3.0 mm or lower.

Moreover, if the thickness Tb of the outer apex portion 10B is less than7.0 mm, the ability to reduce the deformation or distortion of the outerpiece portion 9 o of the bead reinforcing layer 9 cannot be sufficientlydemonstrated. In contrast, if the thickness Tb exceeds 13.0 mm, the beadapex rubber 10 is enlarged, which causes an increase in weight and cost.In view of the respects, the lower limit of the thickness Tb of theouter apex portion 10B is more preferably 10.0 mm or more and the upperlimit thereof is more preferably 12.0 mm or lower.

In particular, the lower limit of the ratio of the thickness Ta to thethickness Tb (Ta/Tb) is 0.10 or more, and more preferably 0.15 or more,and the upper limit thereof is 0.35 or lower, and more preferably 0.25or lower. This enables to improve the flexural rigidity of the beadportion 4 and the ability to alleviate distortion in a well balancedmanner.

Moreover, by covering the auxiliary turned-up portion b2, the bottompiece portion 10A1 formed of a high complex elasticity modulus rubbereffectively prevents, with the auxiliary cord layer 8, the spring backof the auxiliary turned up portion b2, and can prevent breakage at theouter end. In view of the respects, with respect to the bottom pieceportion 10A1, the thickness Tc on an imaginary radial line X2 passingthrough the cross section center G of the bead core 5 as illustrated inFIG. 5 is adjusted to at least 1.0 mm or more, preferably 2.0 mm ormore, and more preferably 2.5 mm or more. In contrast, if the thicknessTc of the bottom piece portion 10A1 is excessively large, the rubbervolume of the outer apex portion 10B decreases. Therefore, the upperlimit thereof is 10.0 m or lower, preferably 7.0 mm or lower, and morepreferably 5.0 mm or lower.

A cushion rubber 20 is provided between the inner apex portion 10A andthe bead cores 5 and between the bead core 5 and the main turned-upportion 6 b 1. For the cushion rubber 20, it is preferable to use a lowcomplex elasticity modulus rubber whose complex modulus of elasticity islower than that of the inner apex portion 10A. For example, a rubberwhose complex modulus of elasticity is about 5.0 to 10.0 MPa ispreferable. This prevents direct contact between the outer end 6 bt ofthe auxiliary turned-up portion 6 b 2 and the high complex elasticitymodulus inner apex portion 10A, and thus, effectively alleviates thedistortion which acts on the outer end 6 bt.

Next, as enlargedly illustrated in FIG. 2, the bead portion 4 isprovided with a chafer rubber 12. The chafer rubber 12 is composed of abase portion 12 a which extends along a sheet surface J1 of the rim Jthrough the radially inner side of the intermediate portion 9 a of thebead reinforcing layer 9, an inner raised portion 12 i which extendsradially outwardly from the end portion at the toe side of the baseportion 12 a, and an outer raised portion 12 o which extends radiallyoutwardly along the outer piece portion 9 o of the bead reinforcinglayer 9 from the end portion at the heel side of the base portion 12 a.The base portion 12 a and the outer raised portion 12 o are arranged insuch a manner as to be in contact with the bead reinforcing layer 9.Moreover, the outer raised portion 12 o extends radially outwardlybeyond the outer end of the flange of the rim J. Such a chafer rubber 12needs a sufficient wear-resistant property and hardness. Thus, such ahard rubber material is used as to have a JISA hardness of preferably60°, and more preferably 70° or more.

A sidewall rubber 13 softer than the chafer rubber 12 is connected tothe radially outer side of the outer raised portion 12 o of the chaferrubber 12, and an inner liner rubber 14 to be provided to the inside ofthe body portion 6 a is connected to the inner raised portion 12 i ofthe chafer rubber 12.

Next, as illustrated in FIG. 6, the bead portion 4 is provided with aheel cover layer 15 composed of at least one cord ply 15A having anorganic fiber cord at least one portion of the outer side of a heel areaAh defined below. The heel area Ah is defined as an area sandwichedbetween a first straight line L1 which extends radially inwardly in adirection perpendicular to the bottom surface 4B of the bead portion 4from the cross section center G of the bead core 5 and a second straightline L2 which extends axially outwardly in a direction perpendicular tothe first straight line L1 from the cross section center G of the beadcore 5. It should be that the bottom surface 4B of the bead portion 4refers to a portion in contact with a linear sheet surface 31 of the rimJ.

In this example, the heel cover layer 15 is disposed including theentire outer side of the heel area Ah. The heel cover layer 15continuously extends in the circumferential direction of the tire, andcovers the outer sides of the base portion 12 a of the chafer rubber 12and the outer raised portion 12 o.

With the wind bead structure, when a tensile force is applied to thebody portion 6 a of the carcass ply 6A during driving, a radiallyoutward force and a moment around the cross section center G of the beadcore 5 are strongly applied to the bead core 5. Thus, the bead core 5 islikely to move to the rim flange side. In accordance with such movementof the bead core 5, the movement of the body portion 6 a also becomeslarge. As a result, stress concentrated at the outer end ft of the outerpiece portion 9 o of the bead reinforcing layer 9 increases to therebypromote rubber separation. Moreover, with the movement of the bead core5, the chafer rubber 12 particularly in the heel area Ah is sandwichedbetween the bead core 5 and the rim J to be compressed in the thicknessdirection, while causing extension in a direction perpendicular to thethickness direction. The extension causes distortion occurring at theinterface between the bead reinforcing layer 9 and the chafer rubber 12,which induces rubber separation.

In contrast, the heel cover layer 15 is unified with the base portion 12a and the outer raised portion 12 o of the chafer rubber 12. Therefore,the extension of the chafer rubber 12 in a direction perpendicular tothe thickness direction can be suppressed. This suppresses separationbetween the bead reinforcing layer 9 and the chafer rubber 12. Moreover,as a result that the compression deformation of the chafer rubber 12 inthe thickness direction is suppressed by the suppression of theextension, the movement of the bead core 5 toward the rim flange issuppressed, whereby the movement of the body portion 6 a of the carcassply 6 is reduced. Furthermore, since the heel cover layer 15 is unifiedwith the chafer rubber to thereby improve the flexural rigidity, beaddeformation can also be suppressed, which combines with the reduction inthe movement of the body portion 6 a to suppress the rubber separationat the outer end ft of the outer piece portion 9 o.

Such a heel cover layer 15 is formed of at least one cord ply 15A inwhich organic fiber cords are arranged. A steel cord shows low adhesionproperties with rubber and does not provide sufficient ductility.Therefore, if the steel cord is used as a cord ply of the heel coverlayer 15, another breakage starting at the heel cover layer 15 arises.Thus, the steel cord is not preferable. As the organic fiber cord, forexample, nylon, rayon, polyester, or aramid is preferably used.Moreover, as the heel cover layer 15, an organic fiber cord having athickness of 1800 to 2200 dtex is preferable, and a ply obtained bydriving 20 to 30 cords per 5 cm is particularly preferable.

FIG. 7 is a side view of the bead portion 4 as viewed from the axiallyouter side. Organic fiber cords 15 c of the heel cover layer 15 arearranged at an angle γ of 30 to 90° relative to the circumferentialdirection of the tire. If the angle γ is less than 30°, opening betweenthe cords 15C is likely to occur, making it impossible to effectivelysuppress the extension in the radial direction of the chafer rubber 12.In view of the respects, the angle γ is preferably 40 to 90°, and morepreferably 45 to 90°. In order to, in particular, increase theprotective effect of the chafer rubber 12, it is preferable that theheel cover layer 15 be composed of two cord plies 15A and 15B which aresuperimposed in such a manner that the organic fiber cords 15C crosseach other as illustrated in FIG. 8.

The heel cover layer 15 may be provided at a portion of the heel areaAh. In this case, it is preferable that the heel cover layer 15 bedisposed to occupy at least 50% or more, more preferably 60% or more,and still more preferably 80% or more of the heel area Ah. Inparticular, it is preferable that the heel cover layer 15 be disposedthroughout the heel area Ah as in this example. This more reliablyimproves the durability of the bead portion 4.

Moreover, in this example, a radially inner end 15 i of the heel coverlayer 15 terminates short of the toe end 4 t of the bead portion 4.Thus, the toe side of the bottom surface 4B of the bead portion 4 canhave its rubber portion brought into contact with the rim. Thisincreases the ability to maintain the internal pressure and alsoeffectively alleviates the impact during driving with the thick rubberportion at the toe side, which helps prevent the development ofoscillation. In order to effectively demonstrate such an effect, thedistance X along the sheet surface J1 between the inner end 15 i of theheel cover layer 15 and the toe end 4 t is 5 mm or more, and preferablyin the range of 10 to 20 mm.

FIG. 9 illustrates another example of the heavy-load tire 1A accordingto the first aspect of the invention. The heavy-load tire 1A of FIG. 9is substantially the same in the structure as the above-described tire1A except that the heel cover layer 15 is not provided. In this case,the above-described effects provided by the heel cover layer 15 cannotbe expected. However, the bead apex rubber 10 having the inner apexportion 10A having an L-shaped cross section sufficiently secures theflexural rigidity of the bead portion and also can alleviate the sheardistortion which acts on the outer piece portion 9 o of the beadreinforcing layer 9. As a result, the rubber separation at the outer endft of the outer piece portion 9 ois suppressed to thereby improve thebead durability while maintaining or improving driving stability.

Next, FIGS. 10 and 11 illustrate an example of a heavy-load tire 1Baccording to a second aspect of the invention. The heavy-load tire 1B ofFIG. 10 or 11 is substantially the same in the structure as theabove-described tire 1A except that a conventional bead apex rubber 30is provided in place of the bead apex rubber 10 having the inner apexportion 10A having an L-shaped cross section. In this case, although theabove-described effects provided by the bead apex rubber 10 cannot beexpected, the above-described effects provided by the heel cover layer15 are demonstrated. Therefore, separation between the bead reinforcinglayer 9 and the chafer rubber 12 is suppressed, and also the sheardistortion at the outer end ft of the outer piece portion 9 o isalleviated to thereby suppress rubber separation, whereby beaddurability is improved.

The bead apex rubber 30 is composed of an inner apex portion 30A whichis formed of a high complex elasticity modulus rubber and is disposed atthe radially inner side and an outer apex portion 30B which is formed ofa rubber whose complex elasticity modulus is lower than that of therubber of the apex portion 30A and is disposed at the radially outerside. The inner apex portion 30A is formed to have a triangular-shapedcross-section which rises from the radially outer side of the auxiliaryturned-up portion 6 b 2.

While description has been made of one particularly preferableembodiment of the present invention, the illustrated embodiment shouldnot be construed as to limit the scope of the present invention; variousmodifications are possible without departing from the scope of thepresent invention.

[Test A]

Experimental heavy-load tires (size: 11R22.5) having the basic structureof FIG. 9 and the specifications shown in Tables 1 and 2 were prepared,and were tested for the following performances. Each tire has the samespecifications other than those shown in Tables 1 and 2. In each tire,the complex modulus of elasticity E*1 of the inner apex portion was 50.0MPa and the complex modulus of elasticity E*2 of an outer apex portionwas 4.0 MPa. It should be noted that Comparative Example A1 is equippedwith a bead apex rubber illustrated in FIG. 12. The test method is asfollows.

<Bead Durability>

A test tire was made to drive at a velocity of 20 km/h on a drum testerunder the conditions: a rim of 7.50×22.5, an internal pressure of 700kPa, and a longitudinal load of 3 times higher than 27.25 kN. Thedriving time taken before damage occurred in the bead portion wasmeasured. Evaluation is expressed as an index in which the driving timeof comparative Example A1 is defined as 100. When the index is larger,the bead durability is more excellent.

<Change in Thickness of Chafer Rubber>

Before and after the above-described bead durability test, the thicknesst (denoted by a reference character t in FIG. 6) of a chafer rubber onthe normal z drawn from the middle position of the height of the outerside of the bead core to the outer side of the bead portion wasmeasured. Then the reduction of the thickness was calculated. It can besaid that when the reduction is smaller, the durability of the beadportion is higher.

<Bead Durability After Deterioration>

A tire was attached to the rim, charged with an internal pressure (1050kPa), and stored in an oven with a temperature of 80° C. for one week.Thereafter, the internal pressure was adjusted to 700 kPa to carry outthe same bead durability test as the above. Evaluation is expressed asan index in which the driving time of comparative Example A1 is definedas 100. When the index is larger, the bead durability is more excellent.

<Driving Stability>

Using a tire static tester, the ratio: the transverse load/the amount ofhorizontal deflection under the conditions: a rim of 7.50×22.5, aninternal pressure of 800 kPa, a longitudinal load of 26.7 kN, atransverse load of 2.0 kN was measured as a horizontal spring constant.The measurement values were expressed as the index in which comparativeExample A1 was defined as 100. when the numerical values are larger, thehorizontal spring constant is higher and the driving stability is moreexcellent.

The test results are shown in Table 1.

TABLE 1 Comparative Examples Examples A1 A2 A3 A4 A5 A6 A1 A2 A3 <Beadreinforcing layer> Number of ply(s) 1 1 1 1 1 1 1 1 1 (sheet) Angle α ofsteel 50 25 35 75 90 50 50 40 70 cord (°) Strength of steel 900 900 900900 900 500 900 900 900 cord (N) Height ho of outer 30 30 30 30 30 30 3030 30 piece portion (mm) Cross section shape Triangular L shape L shapeof inner apex portion shape Thickness Ta of raised 6.5 2.0 2.0 2.0 2.02.0 2.0 2.0 2.0 piece portion (mm) Height Ha (mm) 45 45 45 45 45 45 4545 45 Thickness Tb of outer 6.5 11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0apex portion (mm) Height Hb (mm) 70 70 70 70 70 70 70 70 70 Ratio(Ta/Tb) 1 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 <Test results> Beaddurability 100 100 100 100 80 100 120 120 110 (index) Change inthickness 100 130 120 115 120 115 90 95 90 of chafer rubber (%) Beaddurability after 100 90 95 100 90 90 120 120 110 deterioration (index)Driving stability 100 100 100 100 90 90 110 105 105 (index) Examples A4A5 A6 A7 A8 A9 A10 A11 <Bead reinforcing layer> Number of ply(s) 1 1 1 11 1 1 1 (sheet) Angle α of steel 50 50 50 50 50 50 50 50 cord (°)Strength of steel 700 900 900 900 900 900 700 1200 cord (N) Height ho ofouter 30 40 30 30 30 30 30 30 piece portion (mm) Cross section shape Lshape of inner apex portion Thickness Ta of raised 2.0 2.0 1.0 3.5 2.02.0 0.5 2.0 piece portion (mm) Height Ha (mm) 45 45 45 45 45 45 45 45Thickness Tb of outer 11.0 11.0 10.0 10.0 8.0 7.0 10.0 6.0 apex portion(mm) Height Hb (mm) 70 70 70 70 70 70 70 70 Ratio (Ta/Tb) 0.18 0.18 0.100.35 0.25 0.29 0.05 0.33 <Test results> Bead durability 108 113 105 108107 106 100 100 (index) Change in thickness 95 90 90 90 90 90 90 90 ofchafer rubber (%) Bead durability after 105 110 103 105 103 103 100 100deterioration (index) Driving stability 110 110 110 110 110 110 100 100(index)

Next, comparison was made of bead durability and of driving stabilityusing the tire of Example A1 of Table 1 as a reference while changingonly the complex modulus of elasticity E*1 of the inner apex portion. Itshould be noted that the complex modulus of elasticity E*2 of the outerapex portion of each tire was standardized to 4.0 MPa.

TABLE 2 Examples A12 A13 A14 A15 A16 Complex modulus of 70 30 20 10 80elasticity Bead durability (index) 120 120 100 100 120 Driving stability(index) 110 110 105 105 110

The test results confirmed that the tires of the examples weresignificantly improved in the durability of the bead portion.

[Test B]

Experimental heavy-load tires (size: 11R22.5) having the basic structureof FIG. 10 and the specifications shown in Table 3 were prepared, andwere tested for the following performances. The specifications which arenot illustrated in Table 3 are the same in each example. In Table 3,“polyester” and “nylon” are as follows.

Polyester: Polyester cord (Fineness 2000 dtex, Ends: 25 pieces/5 cm)

Nylon: Nylon cord (Fineness: 2000 dtex, Ends: 25 pieces/5 cm)

The test method is as follows.

The test of the bead durability, the change in the thickness of a chaferrubber, and the bead durability after deterioration were carried out inthe same manner as in the test A. Evaluation is performed using an indexin which comparative Example B1 is defined as 100.

TABLE 3 Comparative Examples Examples B1 B2 B3 B4 B1 B2 B3 B4 B5 B6<Bead reinforcing layer> Number of ply(s) 1 1 0 1 1 1 1 1 1 1 (sheet)Angle α of steel 25 40 — 40 40 40 40 40 55 70 cord (°) Strength of steel900 900 — 900 900 900 900 900 900 900 cord (N) Height ho of outer 30 30— 30 30 30 30 30 30 30 piece portion (mm) <Heel cover layer> Number ofply(s) 0 0 1 1 1 1 1 1 1 1 (sheet) Cord material — — Poly- Poly- Poly-Poly- Poly- Poly- Poly- Poly- ester ester ester ester ester ester esterester Cord angle γ(°) — — 90 25 90 60 45 30 90 90 <Test results> Beaddurability 100 102 90 100 105 108 110 105 102 101 (index) Change inthickness 100 98 120 98 90 85 80 87 93 95 of chafer rubber (%) Beaddurability after 100 105 60 100 110 115 120 115 105 105 deterioration(index) Examples B7 B8 B9 B10 B11 B12 B13 B14 B15 <Bead reinforcinglayer> Number of ply(s) 1 1 1 1 1 1 1 1 1 (sheet) Angle α of steel 40 4040 40 55 25 90 70 55 cord (°) Strength of steel 900 900 900 700 1200 900900 900 900 cord (N) Height ho of outer 30 15 40 30 30 30 30 30 30 pieceportion (mm) <Heel cover layer> Number of ply(s) 1 1 1 1 1 1 1 1 1(sheet) Cord material Poly- Poly- Poly- Poly- Poly- Poly- Poly- Poly-Poly- ester ester ester ester ester ester ester ester ester Cord angleγ(°) 90 90 90 90 90 45 45 45 45 <Test results> Bead durability 105 102105 101 105 105 101 110 110 (index) Change in thickness 90 95 90 95 9090 90 80 80 of chafer rubber (%) Bead durability after 110 105 110 105110 110 105 120 120 deterioration (index)

1. A heavy-load tire comprising: a carcass consisting of one carcass plyhaving a body portion extensive from a tread portion through a side wallportion to a bead core in a bead portion and a turned-up portion whichextends from the body portion, which is turned up from an axially innerside to an axially outer side over the bead core, and which has anauxiliary turned-up portion; a bead reinforcing layer which is disposedat the bead portion and has a reinforcing cord; and a bead apex rubberwhich is disposed farther radially outwardly than the auxiliaryturned-up portion of the carcass ply and which extends radiallyoutwardly in a tapering manner, wherein: the turned-up portion has: amain turned-up portion which curves along an axially inner side, aradially inner side, and an axially outer side of the bead core; and theauxiliary turned-up portion which extends from the main turned-upportion and extends toward the body portion through a vicinity of aradially outer side of the bead core; the bead reinforcing layer has aU-shaped cross section including: an intermediate portion which extendsfarther radially inwardly than the main turned-up portion and along themain turned-up portion; an outer piece portion which extends from anaxially outer side of the intermediate portion and which extendsradially outwardly away from the turned-up portion along an axiallyouter side of the bead apex rubber; and an inner piece portion whichextends from an axially inner side of the intermediate portion and whichextends radially outwardly along the axially inner side of the bodyportion; the reinforcing cord of the bead reinforcing layer has a cordstrength of 700 to 1200 N and the reinforcing cord inclines at an angleof 40 to 70° relative to a circumferential direction of the tire at theouter piece portion; the bead apex rubber has: an inner apex portionwhich is formed of a high complex elasticity modulus rubber and isdisposed at a radially inner side; and an outer apex portion which isformed of a rubber whose complex elasticity modulus is lower than thatof the rubber of the inner apex portion and is disposed at a radiallyouter side; the inner apex portion has an L-shaped cross-sectionincluding: a bottom piece portion along a radially outer side of theauxiliary turned-up portion; and a raised piece portion which rises atan axially inner end side of the bottom piece portion and extendsradially outwardly in a tapering manner along the body portion of thecarcass ply; a base line X1 is defined as a line orthogonal to an outerside of the tire through a point P which is disposed at an axially outerside of the bead apex rubber and has a distance of 25 mm radiallyoutwardly from a bead base line; on the base line X1, a thickness Tb ofthe outer apex portion is 7.0 to 13.0 mm and a thickness Ta of theraised piece portion of the inner apex portion is 1.0 to 4.0 mm; and theouter piece portion of the bead reinforcing layer contacts the point P,and both the outer piece portion and inner piece portion of the beadreinforcing layer extend radially outward from the base line X1.
 2. Theheavy-load tire according to claim 1, wherein: a radial height Hb from abead base line at a radially outer end of the outer apex portion is 40to 100 mm; and a radial height Ha from the bead base line at a radiallyouter end of the raised piece portion of the inner apex portion is notless than 35 mm and not more than the height Hb.
 3. The heavy-load tireaccording to claim 1, wherein a Ta/Tb ratio of the thickness Ta of theraised piece portion to the thickness Tb of the outer apex portion is0.10 to 0.35.
 4. The heavy-load tire according to claim 1, wherein acomplex modulus of elasticity E*1 of the inner apex portion is 20 to 70MPa, and a complex modulus of elasticity E*2 of the outer apex portionis 2.0 to 6.0 MPa.
 5. The heavy-load tire according to claim 1, whereina radial height ho from the bead base line at a radially outer end ofthe outer piece portion of the bead reinforcing layer is greater than 25mm and less than or equal to 40 mm.
 6. The heavy-load tire according toclaim 1, wherein: the bead portion is provided with a heel cover layercomposed of one or more cord plies having an organic fiber cord at atleast one portion of an outermost side of a heel area defined below; theorganic fiber cord of the heel cover layer is arranged at an angle γ of30 to 90° relative to a circumferential direction of the tire; and theheel area is defined as an area sandwiched between a first straight lineextending radially inwardly in a perpendicular manner to a bottomsurface of a bead portion from a cross section center of the bead coreand a second straight line extending axially outwardly in aperpendicular manner to the first straight line from the cross sectioncenter of the bead core.